Communication devices such as terminals or wireless devices are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Such terminals are enabled to communicate wirelessly in a wireless communication system or a cellular communications network, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
The above terminals or wireless devices may further be referred to as mobile telephones, cellular telephones, laptops, or tablets with wireless capability, just to mention some further examples. The terminals or wireless devices in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated at the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals or wireless devices within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
LTE uses Orthogonal Frequency-Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT)-spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as schematically illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms, cf. FIG. 2.
Furthermore, the resource allocation in LTE is typically described in terms of Resource Blocks (RB), where a resource block corresponds to one slot, e.g. 0.5 ms, in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction, e.g. 1.0 ms, is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
The notion of Virtual Resource Blocks (VRB) and Physical Resource Blocks (PRB) has been introduced in LTE. The actual resource allocation to a terminal, e.g. a UE, is made in terms of VRB pairs. There are two types of resource allocations, e.g. localized resource allocation and distributed resource allocation. In the localized resource allocation, a VRB pair is directly mapped to a PRB pair, hence two consecutive and localized VRB are also placed as consecutive PRBs in the frequency domain. On the other hand, the distributed VRBs are not mapped to consecutive PRBs in the frequency domain; thereby providing frequency diversity for data channel transmitted using these distributed VRBs.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about to which terminals data is transmitted and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI) indicated by the Physical CFI CHannel (PCHICH) transmitted in the first symbol of the control region. The control region also comprises Physical Downlink Control CHannels (PDCCH) and possibly also Physical Hybrid Automatic Repeat Request (HARQ) Indication CHannels (PHICH) carrying ACK/NACK for the uplink transmission.
The downlink subframe also comprises Common Reference Symbols (CRS), which are known to the receiver and used for coherent demodulation of e.g. the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 3.
Random Access
In LTE, as in any communication system, a mobile terminal, e.g. a UE or wireless device, may need to contact the network, e.g. via the eNodeB, without having a dedicated resource in the Uplink (UL), i.e. from UE to base station. To handle this, a random access procedure is available where a UE that does not have a dedicated UL resource may transmit a signal to the base station. The first message of this procedure is typically transmitted on a special resource reserved for random access, e.g. on a Physical Random Access CHannel (PRACH). This channel can for instance be limited in time and/or frequency (as in LTE). See FIG. 4.
The resources available for PRACH transmission is provided to the terminals as part of the broadcasted system information in System Information Block 2 (SIB-2) or as part of dedicated RRC signaling in case of e.g. handover.
The resources consist of a preamble sequence and a time and/or frequency resource. In each cell, there are 64 preamble sequences available. Two subsets of the 64 sequences are defined, where the set of sequences in each subset is signaled as part of the system information. When performing a contention-based random-access attempt, the terminal selects at random one sequence in one of the subsets. As long as no other terminal is performing a random-access attempt using the same sequence at the same time instant, no collisions will occur and the attempt will, with a high likelihood, be detected by the base station, e.g. the eNodeB.
In LTE, the random access procedure may be used for a number of different reasons. Among these reasons are                Initial access for e.g. UEs in the RRC_IDLE state;        Incoming handover;        Resynchronization of the UL e.g. UE inactivity after long DRX cycle, e.g. 640 ms, UE transmission after long inactivity, e.g. after 500 ms;        Scheduling request for e.g. a UE that is not allocated any other resource for contacting the base station;        Positioning e.g. for UE performing UE Rx-Tx time difference measurement, for enabling eNode B to perform eNode B Rx-Tx time difference measurement, timing advance etc.;        
The contention-based random access procedure used in LTE Rel-10 is illustrated in FIG. 5 involving a UE and an LTE RAN comprising a base station e.g. an eNode B (eNB) communicating with the UE. The UE starts the random access procedure by randomly selecting one of the preambles available for contention-based random access. The UE then transmits the selected random access preamble on the Physical Random Access CHannel (PRACH) to the base station, e.g. the eNode B (eNB), in the RAN.
The RAN, e.g. the base station, acknowledges any preamble it detects by transmitting a random access response (MSG2) including an initial grant to be used on the uplink shared channel, a Temporary C-RNTI (TC-RNTI), and a Time Alignment (TA) update based on the timing offset of the preamble measured by the eNodeB on the PRACH. The MSG2 is transmitted in the DL to the UE using the PDSCH and its corresponding PDCCH message that schedules the PDSCH contains a Cyclic Redundancy Check (CRC) which is scrambled with the RA-RNTI.
When receiving the response from the RAN the UE uses the grant to transmit a message (MSG3) that in part is used to trigger the establishment of radio resource control and in part to uniquely identify the UE on the common channels of the cell. The timing alignment command provided in the random access response is applied in the UL transmission in MSG3.
In addition, the base station, e.g. the eNB, may also change the resources blocks that are assigned for a MSG3 transmission by sending an UL grant to the UE that has its CRC scrambled with the TC-RNTI which was included in MSG2. In this case the PDCCH is used, to transmit the DCI containing the uplink grant.
The RAN, e.g. the base station, sends a contention resolution message to the UE in MSG4. The MSG4, which is then contention resolving, has its PDCCH CRC scrambled with the C-RNTI if the UE previously has a C-RNTI assigned. If the UE does not have a C-RNTI previously assigned, the MSG4 has its PDCCH CRC scrambled with the TC-RNTI obtained from MSG2.
The procedure ends with the RAN, e.g. the base station, solving any preamble contention that may have occurred for the case that multiple UEs transmitted the same preamble at the same time. This can occur since each UE randomly selects when to transmit and which preamble to use. If multiple UEs select the same preamble for the transmission on RACH, there will be contention between these UEs that needs to be resolved through the contention resolution message (MSG4). The case when contention occurs is illustrated in FIG. 6, where two UEs denoted UE1 and UE2 transmit the same preamble p5 at the same time. A third UE denoted UE3 also transmits at the same RACH, but since it transmits with a different preamble p1 there is no contention between this UE UE3 and the other two UEs UE1 and UE2.
The UE can also perform non-contention based random access. A non-contention based random access or contention free random access can e.g. be initiated by the base station, e.g. the eNB, to get the UE to achieve synchronisation in UL. The eNB initiates a non-contention based random access either by sending a PDCCH order or indicating it in an RRC message. The latter of the two is used in case of handover (HO).
The procedure for the UE to perform contention free random access is illustrated in FIG. 7 involving a UE and an LTE RAN comprising a base station e.g. an eNode B (eNB) communicating with the UE. Similar to the contention based random access schematically illustrated in FIG. 5, the MSG2 is transmitted in the DL to the UE and its corresponding PDCCH message CRC is scrambled with the RA-RNTI. The UE considers the contention resolution successfully completed after it has received MSG2 successfully.
For the contention free random access as for the contention based random access does the MSG2 contain a timing alignment value. This enables the eNB to set the initial/updated timing according to the UEs transmitted preamble.
Dual Connectivity
A dual connectivity framework is currently being considered for LTE Rel-12. Dual Connectivity refers to the operation where a given UE consumes radio resources provided by at least two different network points, e.g. by a Master eNB (MeNB), sometimes herein also referred to as a Main eNB, and a Secondary eNB (SeNB) connected with non-ideal backhaul while in RRC_CONNECTED mode. By the expression non-ideal backhaul when used herein is meant that exchange of messages between the MeNB and SeNB involves at least some delay e.g. 10 ms or more. A UE in dual connectivity maintains simultaneous connections to anchor and booster nodes, where the MeNB is interchangeably called anchor node and the SeNB is interchangeably called booster node. As the name implies, the MeNB controls the connection and handover of SeNB. No SeNB standalone handover is defined for Rel-12. Signaling in MeNB is needed even in SeNB change. Both the anchor node and booster node may terminate the control plane connection towards the UE and may thus be the controlling nodes of the UE.
The UE reads system information from the anchor node. In addition to the anchor node, the UE may be connected to one or several booster nodes for added user plane support. The MeNB and SeNB are connected via the Xn interface, which is currently selected to be the same as the X2 interface between two eNBs.
More specifically Dual Connectivity (DC) is a mode of operation of a UE in RRC_CONNECTED state, where the UE is configured with a Master or Main Cell Group (MCG) and a Secondary Cell Group (SCG). Cell Group (CG) is a group of serving cells associated with either the MeNB or the SeNB. The MCG and SCG are defined as follows:                Master or Main Cell Group (MCG) is a group of serving cells associated with the MeNB, comprising a primary cell, PCell and optionally one or more secondary cells, SCells.        Secondary Cell Group (SCG) is a group of serving cells associated with the SeNB comprising of pSCell (Primary Scell) and optionally one or more SCells.        
Master eNB is the eNB which terminates at least S1-MME. Secondary eNB is the eNB that is providing additional radio resources for the UE but is not the Master eNB.
FIG. 8 describes dual connectivity setup. In this example, only one SeNB is connected to the UE, however, more than one SeNB may serve the UE in general. As shown in the figure, it is also clear that dual connectivity is a UE specific feature and a network node may support a dual connected UE and a legacy UE at the same time.
As mentioned earlier, the anchor and booster roles for any specific node are defined from a UE point of view. This means that a node that acts as an anchor node to one UE may act as booster node to another UE. Similarly, though the UE reads the system information from the anchor node, a node acting as a booster node to one UE, may or may not distribute system information to another UE.
In this disclosure, anchor node and MeNB are used with interchangeable meaning, and similarly, SeNB and booster node are also used interchangeably herein.
MeNB:                Provides system information        Terminates control plane        May terminate user plane        
SeNB:                May terminate control plane        Terminates only user plane        
In one application, dual connectivity allows a UE to be connected to two network nodes to receive data from both nodes to increase its data rate. This user plane aggregation achieves similar benefits as Carrier Aggregation (CA) using network nodes that are not connected by low-latency backhaul connection and/or network connection. Due to this lack of low-latency backhaul, the scheduling and HARQ-ACK feedback from the UE to each of the nodes will need to be performed separately. That is, it is expected that the UE may have two UL transmitters to transmit UL control and data to the connected nodes.
Synchronized or Unsynchronized Dual Connectivity Operation
Since dual connectivity operation involves two non-co-located transmitters, i.e. MeNB and SeNB, one issue related to UE receiver performance is the maximum receive timing difference Δt of the signals from MeNB and SeNB received at the UE receiver. This gives rise to two cases of DC operation with respect to the UE: synchronized DC operation and unsynchronized DC operation.                The synchronized DC operation herein means that the UE may perform DC operation provided the received time difference Δt between the signals received at the UE from the CCs belonging to the MCG and SCG are within a certain threshold e.g. ±33 μs.        The unsynchronized DC operation herein means that the UE may perform DC operation regardless of the received time difference Δt between the signals received at the UE from the CCs belonging to the MCG and SCG i.e. for any value of Δt up to 500 μs.        
Maximum receive timing difference Δt at the UE consists of the components, namely:                (1) Relative propagation delay difference between MeNB and SeNB,        (2) Tx timing difference due to synchronization levels between antenna connectors of MeNB and SeNB, and        (3) Delay due to multipath propagation of radio signalsSCell Activation/Deactivation Procedure        
In dual connectivity the UE will be connected to two eNodeBs simultaneously; MeNB and SeNB. Each of them may have one or more associated SCells which may be configured for DL, or DL and UL CA operation. The SCells are time-aligned to the MeNB and SeNB, respectively, but the MeNB and SeNB may or may not be time aligned with respect to their frame timings and/or their respective System Frame Number (SFN).
MeNB can only activate and deactivate serving cells, e.g. SCells, associated with MeNB. SeNB can only activate and deactivate serving cells, e.g. SCells, associated with SeNB. Cross-eNB activation and/or deactivation is not supported.
The configuration and simultaneous activation, as well as release (hence deactivation), of Special SCell belonging to SeNB is done by MeNB, and hence that the above mentioned agreement shall only refer to SCells associated with MCG and SCG, respectively. Hence, for example, the MeNB configures and activates the Special SCell but not any of the ordinary SCells in the SCG. Similarly the MeNB deactivates and releases the Special SCell but not any of the ordinary SCells in the SCG.
For configuration and simultaneous implicit activation of Special SCell it shall be noted that the activation time may be considerable longer than currently assumed for legacy CA. The fact that the Special SCell goes directly into activated state upon configuration means that the UE might not have had a chance to identify it before the activation, hence the activation might be blind. The UE will also have to acquire SFN timing difference to MeNB by reading MIB from the Special SCell as part of the activation procedure, for purpose of aligning e.g. DRX cycle offset and measurement gap offset between MeNB and SeNB. Acquiring SFN adds maximum an extra 50 ms to the activation time both for regular and blind activation of the Special SCell.
For legacy CA, i.e. CA without dual connectivity, the SCell activation times are 24 and 34 ms for regular and blind activation, respectively; 3GPP TS 36.133 section 7.7, Release 10 (Rel-10). For those numbers to apply it is assumed that the SCell has already been configured by the network via RRC Connection Reconfiguration message (3GPP TS 36.331 section 5.3.5, Rel-10) when the MAC control element activating the cell is received (3GPP TS 36.321 section 5.13, Rel-10). Hence for simultaneous configuration and activation also the RRC procedure delay needs to be taken into account—often 15 ms is assumed for such delay.
Blind activation in legacy CA can make use of that it is known that the maximum time difference between any two cells being aggregated shall be within 30.26 ms (3GPP TS 36.300 annex J.1). Hence the UE only has to assume that the cell to be detected is misaligned by at most half an OFDM symbol, which significantly improves and speeds up the cell detection. In case of unsynchronized MeNB and SeNB both with respect to SFN and frame timing, the UE cannot make such assumption, and the cell detection will be similar to cell detection time for blind handover, which under favourable signal conditions is specified to 80 ms (3GPP TS 36.133 section 5.1).